An important component of many devices, such as solar cells, is its semiconductor junction. In solar cells, a semiconductor junction converts energy from solar radiation into electrical energy. Traditionally, the semiconductor junctions of solar cells have been made from crystalline silicon. Typically, crystalline silicon solar cells are made of silicon wafers having a thickness ranging from 150 to 350 microns. However, silicon-based solar cells are expensive. Silicon has a band gap energy of 1.1 eV, which is at the lower range of effective semiconductors. Furthermore, extracting silicon and removing impurities from it require a large amount of raw material. The process itself consumes large amounts of energy and often results in considerable pollution.
New semiconductor materials and technologies have been developed to relieve solar cell industry's dependency on crystalline silicon and to improve the performance of existing solar cells. In particular, thin film solar cells have shown promising results and attracted considerable attention in recent years. Thin film solar cells are made of semiconductor materials that are often only a few micrometers thick. A typical thin film solar cell comprises two semiconductor layers. The first thin film layer is commonly referred to as the “window” layer or negative type (n-type) semiconductor. The window layer absorbs high energy light energy, but it must also be thin in order to let light pass through the n-type layer to the second semiconductor layer, which is known as the absorbing layer. The absorbing layer or positive type (p-type) layer has a band gap that permits absorption of photons. Less semiconductor material is required in a thin film solar cell, thereby reducing the cost of producing solar cells relative to crystalline silicon solar cells. Thin film photovoltaic cells have been developed using semiconductor materials such as amorphous silicon, cadmium telluride (CdTe), and copper-indium-diselenide (CIS), and copper-indium-gallium-diselenide (CIGS). In particular, CIGS has gained interest and study in recent years. A CIGS-like semiconductor layer can be as thin as a few microns, thus cutting down the cost of solar cell production. In addition, incorporating multiple elements in a semiconductor junction layer such as CIGS creates a graded band gap system, thereby permitting a broader spectrum of solar radiation to be absorbed.
Various processes have been developed to fabricate high efficiency CIGS films for solar cell applications. For example, two processes have been used at the National Renewable Energy Laboratory (NREL). The first involves selenization of a precursor containing copper (Cu), indium (In), and gallium (Ga) by a selenium vapor. Selenization is a process of heating Cu, In or Ga on a substrate in the presence of a selenium gas. A drawback of this process is that selenium-containing gas, such as H2Se, is toxic and presents a health risk to humans in large scale production environments. The second involves growing CIGS from a Cu-rich precursor.
Recently, high efficiency CIGS-based solar cells have been made from (In/Ga)2Se3 precursor film based on a vacuum-based three-stage process. The three stage process is advantageous over previous approaches in that, at each stage, the depositing speed of each element may be controlled by stoichiometry. During the first stage, In, Ga, and Se are co-evaporated and deposited on a molybdenum-coated substrate made of soda lime glass. The co-evaporated elements combine and form a precursor film comprising (InxGa1-x)2Se3. During the second stage, In and Ga depositions are stopped. Instead, only Se is co-evaporated with a new element, Cu, to further coat the substrate which passes through the first stage. At the second stage, the (InxGa1-x)2Se3 precursor is exposed to a flux of Cu and Se with a [Se]/[Cu] ratio around three. Sufficient Cu is added in the second stage to bring the film composition into the range 1.1<[Cu]/([In]+[Ga])<0.95. At the third stage, the remaining In and Ga is then co-deposited with Se in order to convert any excess Cu and Se into CIGS, to adjust the Ga content at the surface, and to construct a surface with potentially beneficial phases containing less Cu than the bulk. Finally, the films are cooled in a flux of Se to temperature around 350° C. The resulting p-type films are further processed to form solar cells by depositing additional layers including CdS, resistive ZnO, or conductive n-type ZnO:Al.
The CIGS films made based on the three stage process are smoother than other known processes and have large grains in the CIGS. They also, in general, have higher efficiency in converting solar radiation into electrical energy than CIGS manufactured by other methods. However, careful stoichiometry control during the three stage process is not enough to eliminate variations in quality, and such variations are still found in the CIGS films fabricated by this method. Such variations are almost inherent to CIGS films due to the multiple elements which are used in the process. Due to the different physical and chemical properties of the different elements, temperatures vary from stage to stage. For example, the first stage requires that the co-deposition of In, Ga, and Se be performed between 250° C. and 500° C. At the second stage and most of the third stage, process temperature has to be greater than 540° C. in order to facilitate CIGS formation. The final step takes place around 550° C. Temperature, batch material purity, and flux rate may result in inconsistency in energy conversion efficiency of the CIGS product. In addition, substrate material, shape, and size as well as back-electrode material and the methods of depositing back-electrode material may all have effect on the efficiency of the final CIGS product.
Given the above background, what is needed in the art are systems and methods to ensure the production quality of films used in solar cells.
Discussion or citation of a reference herein will not be construed as an admission that such reference is prior art to the present application.
Like reference numerals refer to corresponding parts throughout the several views of the drawings. Dimensions are not drawn to scale.